The present invention relates to a novel construction for a ring laser angular rate sensor and more particularly, to a construction which is less costly to manufacture than prior art constructions.
Ring laser angular rate sensors are well known and are particularly described in U.S. Pat. No. 3,373,650, issued to Killpatrick, and U.S. Pat. No. 3,390,606, issued to Podgorski, both of which are assigned to the assignee of the present invention. The above-referred to patents are incorporated herein by reference thereto. Ring laser angular rate sensors of the type referred to commonly utilize a block of material that is dimensionally stable, both thermally and mechanically. The block usually includes a plurality of interconnected gas containing tunnels or passages which form a closed-loop path in the shape of a triangle, a rectangle, or any polygonal path. At each intersection of a pair of interconnected tunnels is a mirror mounted on the block. This arrangement of mirrors and interconnected tunnels forms an optical closed-loop path. Further, at least one anode and one cathode are each mounted on the block and in communication with the gas. Each of the components,including the mirrors, anode, and cathode, must be sealed to the block to form a gas tight seal. The block is usually filled with a lasing gas such as a mixture of helium and neon. A sufficiently large electrical potential is applied between the anode and cathode to cause a discharge current therebetween which results in the production of a pair of counter-propagating laser beams within the block.
Associated with ring laser angular rate sensors is a source of error usually referred to as "lock-in." The source of error is thought to be predominantly caused by back scattering of light at each of the mirrors which form in part the optical closed-loop path which the counter-propagating laser beams traverse. As is well understood by those skilled in the art, there are two widely used techniques applied together to minimize the lock-in error. The first technique consists of dithering the block as taught in U.S. Pat. No. 3,373,650. Mechanically dithering the laser block reduces the source of error caused by lock-in to acceptable levels such that ring laser angular rate sensors have become commercially successful. The second technique consists of producing mirror assemblies structured so as to provide highly polished substrates having superior reflective coatings which achieve minimal laser beam scattering at the surfaces thereof. Development of the mirror assemblies over the years has made it possible for high performance ring laser angular rate sensors.
Prior art mirror assemblies comprise a block of material suitably polished for a mirror substrate. The mirror substrate usually is the same material as the laser block material so that they have matched thermal coefficients of expansion. Commonly, the mirror assembly further comprises alternating layers of a high dielectric material, for example, titanium dioxide (TiO.sub.2) and a lower dielectric material, for example, silicon dioxide (SiO.sub.2), deposited on the mirror substrate by a deposition process such as e-beam deposition or an ion-beam sputtering process, or any other appropriate process to achieve high grade mirrors.
The mirror assemblies of the prior art are usually fixed to the laser block by what is referred to as an optical contact. The optical contact technique requires that the block and the mirror substrate be highly polished so as to form an optical contact when the mirror substrate is pressed against the block. The joining of the laser block and the mirror block by optical contact is usually accomplished at room temperatures.
The mirror assemblies referred to above include a substrate in the form of a block of material having a reflective surface deposited thereon such as the titanium dioxide variety described above. Further, the mirror assemblies may include transducers for controlling optical path length, alignment, and the like. Mirror transducers may be like those shown in U.S. Pat. No. 3,581,227, issued to Podgorski, and assigned to the assignee of the present invention, U.S. Pat. No. 4,383,763, issued to Hutchings et al, U.S. Pat. No. 4,160,184, issued to Ljung, and UK patent application GB 2,104,283 in the name of Litton Systems, Inc. The just above-referred to patents being incorporated herein by reference.
Ring laser sensors of the kind referred to above further include a plurality of electrodes including anodes and cathodes of various constructions like that shown in U.S. Pat. No. 4,007,431, issued to Abbink et al and herein incorporated by reference.
These prior art ring laser angular rate sensors have been proven highly satisfactory in operation and are rapidly gaining wide-spread acceptance for certain applications. These prior art ring laser angular rate sensors, however, are costly to manufacture.
Ring laser angular rate sensors demand dimensionally stable material for many of the parts, and specifically the laser block and the mirror assemblies. This is so, since a closed-loop optical path has only so much leeway in position relative to the tunnels of the cavity and size of the mirrors. The ring laser assembly tolerances are far more critical than simple linear (single line tube) lasers. The mirror assemblies usually include a substrate of material which has thermal and mechanical characteristics substantially similar to those of the block. Commonly, the mirror substrate and the block are of the same material. This is so since the mirror substrate and the block would then have identical thermal coefficients of expansion. In order not to introduce another material type between the block and the mirror substrate, mirror substrates are commonly bonded to the block by what is referred to as an optical contact. That is, the mirror mounting surface of the mirror substrate and end surfaces of the laser block are highly polished to provide an optical contact. Since the block and the mirror substrates are commonly of a quartz-like material, polishing of such surfaces is time consuming and expensive.
Others have attempted to bond the mirror substrate to the laser block by other techniques including epoxy, indium seals, and other materials, but such materials, as indicated earlier, introduce other problems which can deleteriously affect the sensor performance and/or life. Particularly, nonuniformly applied bonding materials between the mirror substrate and laser block may lead to poor or non-existing ring laser alignment within the block. Further, the materials may introduce particulate matter which may react with the lasing gas. All of these problems may cause deleterious effects on laser life and/or performance. Although bonding of the electrodes to the laser block is not as big a concern, forming of a gas tight seal and matching of materials is still important. Indium seals have proven satisfactory as a technique of bonding the electodes to the laser block.